Standart way

Note that sudo do-release-upgrade command is also possible to start upgrading in terminal without GUI.

In my case last command was failed with mysterious error:

Could not calculate the upgrade
An unresolvable problem occurred while calculating the upgrade.
This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu
If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

It seemed that error message was about some apt-sources that I need to disable. But disabling didn't help.